Preparation of toughened polystyrene molding materials

ABSTRACT

Toughened vinyl aromatic compounds, in particular high impact polystyrene, are prepared by a continuous process in which the vinyl aromatic compound and the impact modifier, an elastomeric block copolymer, are each prepared separately in a reaction zone by anionic polymerization, with or without heat exchange with the environment and, if required, with the addition of a solvent, and are polymerized until complete conversion is achieved, and the living chain ends are terminated in a manner known per se, and a thermoplastic, toughened molding material is based on a vinyl aromatic compound having a residual content of less than 50 ppm of monomers and less than 100 ppm of oligomers.

The present invention relates to toughened molding materials based onpolymers of styrene and other vinyl aromatic compounds, which have aparticularly low content of low molecular weight byproducts and residualmonomers, and to a process for their preparation based on anionicpolymerization. Molding materials having a residual monomer content ofless than 50 ppm, in particular less than 10 ppm, and containing notmore than 10 ppm of oligomers having molecular weights of less than 1000are preferably obtained.

In the description below, the term styrene is used even when other vinylaromatic compounds are suitable; the present invention can of courseequally be applied to molding materials comprising derivatives ofstyrene, such as alpha-methylstyrene, 2-, 3- or 4-methylstyrene,tert-butylstyrene, 1,1-diphenylethylene or mixtures thereof.

High impact polystyrene (HIPS) has been produced industrially to dateexclusively by free radical polymerization of styrene in the presence ofa dissolved rubber, the rubber itself being the starting point of livingstyrene chains (grafting; cf. DE-A-40 46 718; U.S. Pat. No. 3,903,202)and thus being compatible with the styrene homopolymers. The processesusually employed in industry are very expensive and are usually carriedout in a plurality of reaction apparatuses in succession, for example ina 2-kettle 2-tower cascade or in a 3-tower cascade according to theabove documents. Furthermore, the use of horizontal reactors (DE-A-24 44650) has already been proposed.

All free radical polymerization processes have the disadvantage that thepolymerization is incomplete and relatively large amounts of oligomersare formed as byproducts and have to be separated off together with theresidual monomers after the polymerization in an expensive purificationprocess, since especially monomeric styrene, owing to its strong odor,cannot remain in the prepared molding material. A residual content offrom about 100 to 200 ppm of styrene has to date been accepted asunavoidable.

It is known that styrene can be subjected to anionic polymerization(EP-A-176 611; U.S. Pat. No. 3,035,033; FR-A-2 146 581), it beingpossible to carry out polymerization to virtually complete conversion. Arecent overview of the anionic polymerization of, in particular, styreneis given by Priddy (J. Polym. eng. 10 (1991), 334).

In the known anionic polymerization processes, however, only polymerswhich are not toughened can be prepared: since no grafting of the rubberpresent takes place in the anionic polymerization, the rubber cannotenter into any interactions with the surrounding styrene homopolymer(the matrix), and only products having entire inadequate mechanicalproperties are obtained.

It is an object of the present invention, therefore, to find a processfor continuous preparation of toughened molding materials based onpolystyrene which comprise, in a matrix of the styrene homopolymer, anelastomeric styrene-butadiene block copolymer, the target proportion ofbutadiene in the styrene-butadiene block copolymer being from 15 to 80%by weight and in the molding material as a whole being from 2 to 50% byweight.

It is a particular object of the present invention to prepare toughenedmolding materials based on polystyrene and having only a low content ofresidual monomers and oligomers and high thermal stability and goodmechanical properties, which molding materials are at least equivalentto the corresponding molding materials obtained by free radicalpolymerization.

The novel process essentially comprises the following:

styrene is subjected to anionic polymerization in a first reaction zone(I) without heat exchange with the environment and, if required, withthe addition of a solvent, until complete conversion is achieved, andthe living chain ends are terminated in a manner known per se;

in a second reaction zone (II) operated parallel to the first one,styrene and butadiene are subjected to isothermal anionic polymerizationwith the use of a suitable solvent, likewise to complete conversion, togive an elastomeric block copolymer whose living chain ends areterminated with a proton-active substance either immediately or afterappropriate coupling by means of a polyfunctional compound;

the two polymer streams are combined in a mixing zone (III) and togetherfreed from solvent and volatile components in a further zone(devolatilization zone).

It is understood that the novel process is, in particular, operatedcontinuously.

The chosen ratio of the reactants from reaction zones I and II ispreferably such that the molding material ultimately contains from 4 to25% by weight of polybutadiene.

Preference is likewise given to a process variant in which mixtures ofpolybutadiene and styrene-butadiene block copolymer are prepared inreaction zone II.

The principle of the process is as follows. Two reaction apparatuses orzones I and II operated in parallel are present, zone I being operatedessentially adiabatically, ie. with heating by means of the liberatedenthalpy of reaction and without substantial heat exchange with theenvironment, and zone II being operated essentially isothermally, ie.with cooling and removal of the enthalpy of reaction liberated there.Two polymer streams are produced by an anionic method in these zones andare combined in a mixing zone III. In the adiabatically operated zone I,only styrene (or alpha-methylstyrene etc.) is polymerized, the polymersthereof being intended to form the matrix of the molding material to beprepared. In zone II, the rubber (the soft component), ie. thediene-containing polymer, is prepared. The local addition of monomersand solvent-may vary and depends on the desired molecular weightdistribution and on the intended chemical composition. Mixing iseffected above the phase inversion point, ie. the combined streams arenot homogeneously miscible with one another. With regard to the termsphase inversion and phase inversion point, reference may be made toAngew. Makrom. Chem. 58/59 (1977), 175. Before the polymer streams fromzones I and II enter the mixing zone, the living chain ends areadvantageously terminated by means of proton-active substances.

Each of the reaction zones I and II may consist of one or more stirredkettles, tower reactors or other apparatuses having mechanical stirringelements (for example, loop or circulation reactors) or advantageouslyalso of static apparatuses, such s tube reactors, with or withoutappropriate baffles. Whether a plurality of relatively small tubereactors connected in series or whether large individual apparatuses areused is unimportant. The necessity of baffles (static mixers) thendepends on the viscosity of the reaction mixture and accordingly on thelevel of heat exchange with the tube wall.

Reaction zone I

The tube reactors in which styrene and, separately therefrom, initiator,the latter advantageously in a solvent, are fed in at the top end areparticularly advantageous for the continuous anionic, adiabatichomopolymerization of styrene. In principle, however, all reactants mayalso be premixed immediately before entering the reaction apparatus,especially if it is intended to manage without a solvent. The entrytemperature is advantageously from room temperature to 70° C.,preferably from 30° to 65° C., but it is also possible to use aprecooled solution at, for example, from 0° to 20° C. The outlettemperature of the polymer mixture at the end of the reaction dependsessentially on the amount of solvent added. It is in any case above100°C., preferably from 150° to 360° C. The apparatuses used should bedesigned in such a way that the pressure built up can reach up to about25 bar. In the process described, products having a narrow molecularweight distribution are mainly obtained. In order to obtain a broadermolecular weight distribution, the initiator can be divided up and oneor more portions added in downstream tube sections. The monomer, too,can be added in portions to various tube sections.

The hydrocarbons usually used for anionic polymerization may be employedas solvents. Suitable solvents are aliphatic, cycloaliphatic or aromatichydrocarbons- which are liquid under the reaction conditions and arepreferably of 4 to 12 carbon atoms. Examples are cyclohexane,methylcyclohexane, toluene, ethylbenzene and xylene.

Suitable initiators for the polymerization are the known hydrocarboncompounds of lithium (RLi), where R is an aliphatic, cycloaliphatic,aromatic or aliphatic-aromatic hydrocarbon radical. The hydrocarbonradical may be of 1 to about 12 carbon atoms.

Examples of lithium hydrocarbons are methyllithium, ethyllithium, n- andsec-butyllithium, isopropyllithium, cyclohexyllithium and phenyllithium.n-Butyllithium and sec-butyllithium are particularly preferred.

In order to obtain products having high thermal stability, it isessential to terminate the living chain ends and to do so immediatelyafter the monomer has been completely converted. Termination of theliving chain ends comprises reacting the polymer, still capable offurther reaction, with a proton-active substance, or a Lewis acid ortreating it with a coupling agent.

Examples of proton-active substances and coupling agents are alcohols,water, epoxides, aldehydes, esters, anhydrides, organic acids andhalogen compounds, eg. silicon tetrachloride.

Reaction zone II

In the reaction zone II, block copolymers of styrene and butadiene areprepared. Mixtures of such block copolymers with polybutadiene may alsobe obtained. The preapration of styrene/butadiene block copolymers isknown per se. Suitable reactors are the same apparatuses as describedabove for reaction zone I, but the enthalpy of reaction (the heat ofpolymerization) is removed continuously so that a uniform temperatureprevails. Highly isothermal operation is, however, not absolutelyessential.

Tube reactors in which the heat of reaction is removed either by meansof jacket cooling or by means of internal cooling tubes are particularlysuitable. Typical of the invention is the fact that the preparation ofthe polydiene and/or block copolymer is in any case carried out insolution, in contrast to the reaction zone I. The final concentration ofthe polymer solution at the outlet of reaction zone II is in general notmore than 60%, preferably not more than 50%.

The vinyl aromatics and solvents chosen may be the same compounds asdescribed for reaction zone I.

All hydrocarbons having a conjugated C═C double bond may be used asdienes. Butadiene, dimethylbutadiene, 1-phenylbutadiene, isoprene andpiperylene and mixtures of these compounds are particularly suitable.

It is possible to choose block copolymers having any desired structure.Thus, two-block copolymers, three-block copolymers, multiblockcopolymers or star block copolymers may be prepared in the novelprocess, and the transition between the blocks may be both well definedand tapered. Block copolymers having block segments in which styrene andbutadiene were randomly copolymerized are also particularlyadvantageous.

Star block copolymers are obtained in a known manner by coupling. Thecoupling center is formed by reacting the living anionic chain ends witha polyfunctional coupling agent. Examples of suitable compounds aredescribed in U.S. Pat. Nos. 3,985,830, 3,280,084, 3,637,554 and4,091,053. Epoxidized glycerides, such as epoxidized linseed oil orsoybean oil, are preferably used; divinylbenzene is also suitable.Dichlorodialkylsilanes, dialdehydes, such as terephthalaldehyde, andesters, such as ethyl formate or ethyl benzoate, are particularlysuitable for the dimerization.

The amount by weight of the diene in the total block copolymer is from15 to 80% and the amount of styrene is accordingly from 85 to 20%, thepercentages being based on the monomer combination styrene/butadiene.Butadiene/styrene block copolymers composed of from 25 to 60% ofbutadiene and from 75 to 40% of styrene are particularly preferred.

In order to prepare, for example, a star block copolymer comprisingpolystyrene and polybutadiene blocks, styrene, a solvent, such ascyclohexane, and initiator are fed, separately or in premixed form, tothe top end of a tube reactor, while butadiene is metered in furtherdownstream. The exact metering point of the butadiene depends on thepolymerization rate of the styrene, which can be controlled by means ofthe polymerization temperature, the concentration of the styrene in thesolvent and the residence time in the tube reactor.

In order to obtain sharply separated blocks, the styrene must becompletely polymerized before butadiene is metered in. After theaddition of butadiene, it too is allowed to polymerize completely. Theliving chain ends are then coupled with a conventional coupling agent,for example with silicon tetrachloride, divinylbenzene or apolyfunctional epoxide, by adding the coupling agent at a metering pointfurther downstream. The exact metering point depends on thepolymerization rate of the butadiene.

After the coupling, the living chain ends are terminated by means of aproton-active substance. The temperature for the isothermalpolymerization is advantageously from 20° to 150° C., preferably from 30° to 120° C. Owing to the boiling point of the diene, the reaction iscarried out at superatmospheric pressure. The pressures are in generalfrom 2 to 25 bar. The reaction can be accelerated by adding Lewis bases.Preferred Lewis bases are polar aprotic compounds, such as ether andtertiary amines. Examples of particularly effective ethers aretetrahydrofuran and aliphatic polyethers, such as diethylene glycoldimethyl ether. Suitable tertiary amines are tributylamine and pyridine.The Lewis base is added to the nonpolar solvent in a small amount, forexample from 0.5 to 5% by volume. Tetrahydrofuran in an amount of from0.1 to 0.3% by volume is particularly preferred. Experience has shownthat it is possible to manage with about 0.2% by volume in most cases.

In the mixing zone III, which may consist of any mixing apparatus, ie.both dynamic stirring units and static mixers,the polymer streams fromzones I and II are mixed, and, if required, assistants such aslubricants, antistatic agents, antioxidants, mold release agents, etc.may be added.

The ratio of the flow rates depends on the desired properties of themolding material and is adjusted so that the rubber content, expressedby the polydiene content, is from 2 to 50, preferably from 4 to 25, % byweight.

The mixing temperature is from 100° to 300° C., preferably from 120° to260° C., depending on the temperature of the individual streamintroduced.

A noteworthy feature of the processes in the mixing zone is that thepolymer streams are mixed at above the phase inversion point, ie. theyare not homogeneously miscible and accordingly no phase inversion takesplace; instead, the rubber-containing solution flocculates to a certainextent and the rubber is distributed in the continuous phase.

The polymer blend is then fed to a devolatilization zone and is freedfrom the solvent by the usual means. Owing to the relatively hightemperature required there, the devolatilization zone also performs thefunction of producing sufficient crosslinking of the dispersed rubberparticles; for this purpose, it may be advantageous to provide arelatively long residence time in the devolatilization zone, for exampleat from 220° to 290° C., and possibly also to add a suitable assistant.Inter alia, free radical initiators, such as peroxides, perketals orperesters or C-C labile compounds, such as tetramethyldiphenylethane orhexaphenylethane, are suitable.

EXAMPLE 1

Reaction zone I consists of a tube of V2A stainless steel having aninternal diameter 14 mm and a length of 3000 mm.

The reaction zone II consists of a tube of this type which has aninternal diameter of 44 mm and a length of 3300 mm which is equippedwith a cooling jacket and is filled over its entire length with a staticmixer of type SMX from Sulzer.

The mixing zone (III) consists of a tube of this type which has adiameter of 8 cm and a length of 40 cm and is likewise equipped with astatic mixer.

The solvent used was distilled and was dried over alumina. Styrene wasdistilled and was dried over alumina. Butadiene was dried over amolecular sieve. 1,1-Diphenylethylene was distilled over n-butyllithium.

Unless stated otherwise, the amounts shown below are by weight.

A mixture of 70 parts of styrene with 30 parts of cyclohexane wasmetered in at the top of the reaction tube I at a rate of 6.35 1/h. 150ml/h of a 1% strength solution of sec-butyllithium in cyclohexane weremetered in separately.

After a short induction period, the temperautre at the tube outletincreased to 195° C. The resulting conversion was over 99.99%.

At the same time, 4.3 kg/h of a mixture of 3.72 parts of butadiene, 2.48parts of styrene and 35.1 parts of cyclohexane and, separatelytherefrom, 240 ml/h of a 1% strength solution of butyllithium incyclohexane were fed to the reaction tube II. The reaction temperaturewas 72° C. Under the stated conditions, butadiene polymerized first and,when it had been essentially completely consumed, styrene polymerized. Atapered butadiene/styrene block copolymer was obtained. The solidscontent was 15%.

An excess, based on the amount of initiator used, of carbon dioxide andwater was added to the solutions emerging from the reaction zones I andII before they were fed to the mixing zone, without further heatexchange with the environment. At the end of the mixing zone, a 54%strength mixture of a dispersed rubber phase and homogeneous polystyrenephase was obtained. This mixture was at 140° C. and was devolatilizedand worked up by conventional methods.

EXAMPLE 2

The experiment from Example 1 was repeated, 0.1% of a commercialstabilizer (Irganox 1076 from Ciba-Geigy AG) and 2% of liquid paraffin,based in each case on the amount of polymer in the mixture, also beingadded to the polymer streams from the reaction zones I and II on entryinto the mixer.

EXAMPLE 3

A mixture of 80 parts of styrene and 20 parts of cyclohexane was meteredat a rate of 8.3 1/h into reaction zone I. 240 ml/h of a 1% strengthsolution of sec-butyllithium in cyclohexane were fed in for initiation.The temperature at the end of the reaction tube reached 230° C.

An SBS block copolymer was prepared in reaction zone II. For thispurpose, at the top end, 1.30 1/h of a mixture of 1.75 parts of styrenein 7.0 parts of cyclohexane and 160 ml/h of a 1% strengthsec-butyllithium solution were metered in, and a mixture of 2.5 parts ofbutadiene and 0.75 part of styrene in 13.0 parts of cyclohexane wasmetered in downstream at a rate of 2.45 1/h. The internal temperaturewas kept at 65° C.

The polymer formed was aftertreated with isopropanol.

EXAMPLE 4

The experiment of Example 1 was repeated by using a mixture of styreneand 1,1-diphenylethylene in a ratio of 3:1 instead of styrene inreaction zone I and subsequently metering in downstream 3%, based onthis mixture, of styrene.

Accordingly, the amount of styrene used in reaction zone II was alsoreplaced by a mixture of styrene and 1,1-diphenylethylene in a ratio of3:1, and once again 3% of styrene was subsequently metered indownstream.

The procedure was then continued as described under Example 1.

EXAMPLE 5

The experiment in Example 1 was repeated by dividing the amount ofinitiator used in reaction zone I in a ratio of 2:1 and metering in thelarger amount at the top of the reaction tube and the smaller amountdownstream.

We claim:
 1. A process for continuous preparation of toughened moldingmaterials based on polystyrenes, comprising, in a matrix of a styrenehomopolymer, an elastomeric styrene-butadiene block copolymer, theproportion of butadiene in the styrene-butadiene block copolymer beingfrom 15 to 80% by weight and in the molding materials a whole being from2 to 50% by weight, which comprises:producing a first polymer stream bysubjecting styrene to anionic polymerization in a first reaction zone(I), without heat exchange with the environment and, if required, withthe addition of a solvent, until complete conversion is achieved, andthe living chain ends are terminated in a manner known per se; producinga second polymer stream by subjecting, in a second reaction zone (II)operated parallel to the first reaction zone (I), styrene and butadieneto isothermal anionic polymerization with the use of a suitable solvent,likewise to complete conversion, to give an elastomeric block copolymerwhose living chain ends are terminated with a proton-active substanceeither immediately or after appropriate coupling by means of apolyfunctional compound; and combining the two polymer streams in amixing zone (III) and freeing the combined polymer streams from solventand volatile components in a devolatilization zone.
 2. A process asclaimed in claim 1, wherein the ratio of the reactants from reactionzones I and II is chosen so that the molding material contains from 4 to25% by weight of polybutadiene.
 3. A process as claimed in claim 1,wherein blends of polybutadiene and styrene/butadiene block copolymerare prepared in reaction zone II.
 4. A process as claimed in claim 1,wherein the reaction products from reaction zones I and II are mixedabove the phase inversion point.
 5. A process as claimed in claim 1,wherein a free radical initiator is added to the polymer blend beforeentry into the devolatilization zone.
 6. The process of claim 1, whichadditionally comprises the step of fabricating the combined polymerstreams exiting from the devolatilization zone into a film, sheet, ormolding.
 7. The process of claim 1, wherein styrene is subjected toanionic polymerization in the first reaction zone (I) at an entrytemperature of from 30° to 65° C. to provide a first polymer streamhaving a temperature between 150° C. and 360° C.
 8. A process forcontinuous preparation of toughened vinyl aromatic compounds, whichcomprises:producing a first polymer stream having a temperature above150° C. by subjecting, at an entry temperature of from 30° to 65° C., avinyl aromatic compound to anionic polymerization in a first reactionzone (I), without heat exchange with the environment and, if required,with the addition of a solvent, until complete conversion is achieved,and the living chain ends are terminated in a manner known per se;producing a second polymer stream by subjecting, in a second reactionzone (II) which is operated parallel to the first reaction zone (I), thevinyl aromatic compound and a diene to isothermal anionic polymerizationwith the use of a suitable solvent, likewise to complete conversion, togive an elastomeric block copolymer whose living chain ends areterminated with a proton-active substance either immediately or afterappropriate coupling by means of a polyfunctional compound; andcombining the two polymer streams in a mixing zone (III) and freeing thecombined polymer streams from solvent and volatile components in adevolatilization zone.